Photovoltaic (PV) panels are fabricated with strings of PV cells connected in series to convert solar energy to electric power. In some cases the cells may be arranged as a combination of both parallel and series connections. During the manufacturing process, a variety of tests are used to determine adherence to stringent target specification tolerances on all mechanical as well as electrical aspects of the PV panels to ensure their long-term safety and reliability. These tests include a combination of visual, mechanical, optical and electrical techniques. For high-quality and high-speed manufacturing, many of these techniques are automated using computer vision, robotics and electronic instrumentation.
The electrical performance of a photovoltaic module depends on multiple factors. These factors include temperature, solar irradiance, angle-of-incidence, type of PV-cells, air mass, etc. FIG. 1 illustrates the current-voltage characteristic (I-V curve) of a typical PV panel under certain operating conditions. When the output terminals of the panel are shorted together, the output voltage (V) is zero, and the output current (I) is ISC, which is the short-circuit current generated by the panel. As the output voltage increases, the I-V curve remains at a fairly constant level of current until it reaches a knee at which point it descends rapidly toward zero current at VOC, which is the open-circuit output voltage of the panel.
PV panels are rated under standard test conditions (STC) of solar irradiance of 1,000 W/m2 with zero angle of incidence, solar spectrum of 1.5 air mass and 25° C. cell temperature. PV panels have traditionally been tested by exposing the panel to simulated sunlight under the standard test conditions and collecting enough data to construct an I-V curve. From this data, key specifications may be determined include maximum rated power, open circuit voltage, short circuit current, maximum power voltage, maximum power current, and temperature coefficients. However, standard techniques for exposing a PV panel to 1,000 W/m2 artificial illumination equivalent to sunlight may be prohibitively expensive, time consuming and in many cases impractical. For example, continuously exposing a PV panel to illumination at 1,000 W/m2 may cause heating of the PV cells, thereby distorting the I-V characteristics to be measured for the determination of the panel performance at STC.
To eliminate the problems caused by continuous light sources, “flash” testing techniques have been developed. During flash testing of a PV module, a flash of light, typically 1 to 50 ms long, from a Xenon filled (or equivalent) arc lamp is used. The spectral properties of the arc lamp are controlled to match the spectrum of the sunlight to the extent required. Alternate flash generation technologies can involve a variety of light sources including light-emitting diodes (LEDs). The output from the PV panel in response to the flash is collected by a data acquisition system and processed using a computer to determine the I-V characteristic of the PV panel under test. The results are compared to the target specifications with appropriate tolerances to determine if the PV panel performs within the required specifications. Flash testing of PV panels is possible due to the rapid-response of the photovoltaic cells, and limited charge accumulation and storage requirements before the IV characterization tests can be adequately performed.
PV panels have traditionally been manufactured as independent components that require external power conversion apparatus to optimize the operating point of the panel and/or to convert the DC power generated by the PV panel to AC power for connection to a local utility grid. PV panels are now being fabricated as modules with integral power converters. On a typical PV module with an integral power converter, only the output terminals of the power converter are accessible for testing. The output terminals of the PV cells are sealed to protect against environmental degradation.
Conventional flash testing cannot be used on PV modules with fully integrated power converters such as power optimizers, AC microinverters and/or diagnostic and safety related communication capabilities for several reasons: (1) large inherent energy storage devices in power optimizers, AC microinverters and communication circuits; (2) large startup wattage requirements for allowing reliable startup of the module integrated electronics; (3) algorithmic latencies of maximum power point-tracking and digital control; and/or (4) connect and disconnect requirements as regulated by the standards and utilities.